Apparatus for thermal tuning of path length control drivers

ABSTRACT

A path length control driver apparatus includes a base plate having a first central member having first and second opposite ends, a first outer rim membr and a first flexible annular diaphragm area having first and second opposite sides. The first flexible annular diaphragm area is between and integral with the first central member and the first outer rim member. A central axis passes centrally through the first and second opposite ends of the first central member. A first piezoelectric ceramic material has first and second opposing sides and the first side of the first piezoelectric ceramic material is rigidly affixed to the first flexible annular diaphragm area&#39;s first side. A second piezoelectric ceramic material has first and second opposing sides and the first side of the second piezolectric ceramic material is rigidly affixed to the first flexible annular diaphragm area&#39;s second side. A first annular ring electrode is rigidly affixed to the second side of the first piezoelectric ceramic material, and a second annular ring electrode is rigidly affixed to the second side of the second piezoelectric ceramic material. The first and second electrodes have selected annular ring widths of different sizes to provide a selected temperature sensitivity of movement of the central member along the central axis.

BACKGROUND OF THE INVENTION

The present invention relates to piezoelectric control elements. Moreparticularly, the present invention relates to apparatus for thermaltuning of path length control drivers for ring laser gyros.

Ring laser gyros of the type manufactured by Honeywell Inc. ofMinneapolis, Minnesota are well known. As its name implies, a ring lasergyro is a gyroscope which utilizes a laser beam directed to travel in aclosed path, i.e., a ring, within a ring laser gyro block to detectrotation about the axis of the path around which the laser beam isdirected. The ring laser gyro must be capable of operating over a widerange of temperatures. As a result, the material of which the gyroscopeis made suffers thermal expansion and contraction as the temperaturechanges. The laser beam within the ring laser gyro is directed in itspath by means of mirrors, typically in a triangular path having threemirrors. One mirror is located at each corner of the triangular path.Other types of ring laser gyros having other polygonal shapes, such asfour sided ring laser gyros are also known, and they operate accordingto the same principles as discussed hereinabove. The temperature changeresulting in expansion or contraction, causes a change in the pathlength.

In order to properly operate, ring laser gyros require a laser pathwhich is maintained at a substantially constant length. This isimportant since the laser beam intensity is dependent upon the pathlength. Variations in the beam intensity can adversely affect theperformance parameters of the gyro and such variations can cause gyroerrors. In order to maintain a constant ring laser path length, a mirrortransducer is commonly employed. Such mirror transducers compensate forthermal expansion effects which are inherent in the ring laser gyromembers and which cause undesirable path length variations, by changingthe position of at least one of the mirrors with reference to the ringlaser gyro block. This effect is illustrated in FIG. 12 which shows afirst curve PLC which represents selective movement of a mirrortransducer substrate by a path length control driver, and a second curveC corresponding to path length variances of the ring laser gyro assemblywith temperature. The desired result is to have the path length controldriver force the mirror transducer substrate in an equal and oppositedirection to that of the ring laser gyro assembly's movement as causedby the reaction to temperature changes. This is indicated by the dashedline PLC+C which represents the sum of the thermal movements of the pathlength control driver assembly and the ring laser gyro assembly. Suchpath length driver control effectively cancels any thermal movement ofthe ring laser gyro assembly, thereby maintaining a constant pathlength.

Mirror transducers for path length control in ring laser gyros havegenerally been fabricated with a variety of piezoelectric element driventransducer assemblies. Such assemblies have included one or morepiezoelectric elements. Examples of piezoelectric control elements usedin ring laser gyro applications are illustrated in U.S. Pat. No.3,581,227 issued to Podgorski, U.S. Pat. No. 4,383,763 issued toHutchings et al., U.S. Pat. No. 4,697,323 issued to Ljung, et al., andU.S. Pat. No. 4,488,080 issued to Baumann. A mirror substrate withselected thermal compensation is disclosed by Toth in U.S. Pat. No.4,915,492.

In the '323 patent, as illustrated in FIG. 1, Podgorski shows and claimsthe use of a transducer block 4 composed of a dimensionally stablematerial which is mounted to a ring laser gyro block 40. The transducerblock is circularly grooved on its internal side to leave a depressedthin integral gas impervious annular diaphragm 6 extending between acentral post 5 and an outer rim 9. The central post is generallycylindrical and is inwardly-standing from and integral with the annulardiaphragm. The outer rim is also integral with the annular diaphragm. Astack of piezoelectric ceramic wafers 1 is located in an opening 8 whichis bored into the underside of block 4. The ceramic wafer stack 1 bearsagainst the external side of the annular diaphragm and of the inwardlystanding post 5. The opening containing the ceramic wafer stack isclosed with a rigid disk-like member 2 which supports the stack ofceramic wafers. On the internal side of the central post 5 is a lightreflecting means 7, generally provided by a deposition of selectedmaterials to form a mirror. The transducer assembly is positioned on thelaser block 40 to reflect the laser beams within the cavity provided bythe laser block.

All of the other aforementioned patents utilize one or more of theprinciples taught by Podgorski. Honeywell Inc. of Minneapolis, Minnesotahas long used a double diaphragm mirror assembly which includes apiezoelectric driver assembly. One example of a double diaphragm mirrorassembly is shown in Ljung et al. The mirror assembly includes a centralpost which is coupled to a driver assembly. The driver assembly is acup-shaped metallic driver fixture having an annular diaphragm extendingbetween an integral central member and outer rim member. The centralmember is rigidly coupled to or attached to the central post of themirror assembly. A pair of symmetrical donut-shaped piezoelectric disksare positioned on opposite sides of the annular diaphragm to provide thetransducer action.

Toth in U.S. Pat. No. 4,915,492, which is hereby incorporated byreference, discloses a mirror substrate comprised of a mirror assemblyand a driver assembly. Both the mirror assembly and the driver assemblyinclude a diaphragm portion surrounding an integral central post member.The central post members are rigidly coupled together to provide tandemtranslation movement along a central axis passing through the centralpost members. A pair of non-symmetrical piezoelectric disks arepositioned on opposite sides of the diaphragm portion of the driverassembly. The sizes of the piezoelectric disks are selected to achieve aselected temperature sensitivity of movement of the tandem centralmembers along an axis passing therethrough.

In operation, mirror transducers of the kind described hereinabovegenerally have a quite limited range of movement. Therefore, in ringlaser gyro applications a mode reset circuit is often employed tomaintain the transducer within its operating range. Herein mode isdefined as the equivalent of one wavelength of the laser beam. For ahelium-neon laser, one mode is equal to 6328 microns which is equal to24.91 micro-inches. Temperature changes of the gyro laser block as wellas the transducer assembly, itself, are primary contributors to pathlength changes of the laser beam. Unfortunately, each "mode reset" ofthe transducer contributes to the overall gyro performance error budget.

In the present invention, electrodes are used as compensating elementsof a path length control driver. Using electrodes in this way provides amore reproducible compensation capability. As noted hereinabove,piezoelectric material size or thickness was also used as a compensatingelement. However, it is difficult to obtain constantly reproducibledriver controlled thermal compensation capability by varying thepiezoelectric material thickness. The required thermal compensation fora more easily producible design requires more compensation than thepiezoelectric material sizing technique provides. This is particularlytrue, if mode resets are to be avoided. In one aspect of the instantinvention, the electrode and piezoelectric ceramic material employed bythe invention are analogous to fine and coarse tuning elementsrespectively. Therefore, the electrode and the piezoelectric materialcompliment each other quite well from a thermal compensation viewpoint.

SUMMARY OF THE INVENTION

A path length control driver apparatus in accordance with the presentinvention includes a base plate having a first central member havingfirst and second opposite ends, a first outer rim member and a firstflexible annular diaphragm area having first and second opposite sides.The first flexible annular diaphragm area is between and integral withthe first central member and the first outer rim member. A central axispasses centrally through the first and second opposite ends of the firstcentral member. A first piezoelectric ceramic material has first andsecond opposing sides and the first side of the first piezoelectricceramic material is rigidly affixed to the first flexible annulardiaphragm area's first side. A second piezoelectric ceramic material hasfirst and second opposing sides and the first side of the secondpiezoelectric ceramic material is rigidly affixed to the first flexibleannular diaphragm area's second side. A first electrode is rigidlyaffixed to the second side of the first piezoelectric ceramic material,and a second electrode is rigidly affixed to the second side of thesecond piezoelectric ceramic material. The first and second electrodeshave selected shapes of different sizes to provide a selectedtemperature sensitivity of movement of the central member along thecentral axis.

In yet another aspect of the invention, a path length control apparatusis disclosed comprising a base plate having a first central memberhaving first and second opposite ends, a first outer rim member and afirst flexible annular diaphragm area having first and second oppositesides. The first flexible annular diaphragm area is between and integralwith the first central member and the first outer rim member. A centralaxis passes centrally through the first and second opposite ends of thefirst central member. A first stack comprising a first plurality ofpiezoelectric ceramic material elements has first and second opposingsides and the first side of the first stack of piezoelectric ceramicmaterial is rigidly affixed to the first flexible annular diaphragmarea's first side. A second stack comprising a second plurality ofpiezoelectric ceramic material elements has first and second opposingsides, and the first side of the second stack of piezoelectric ceramicmaterial is rigidly affixed to the first flexible annular diaphragmarea's second side. At least first and second electrodes are included.The first electrode is rigidly affixed to the second side of the firststack of piezoelectric ceramic material. The second electrode is rigidlyaffixed to the second side of the second stack of piezoelectric ceramicmaterial. The at least first and second electrodes have selected shapesin order to provide a selected temperature sensitivity of movement ofthe central member along the first central axis.

A path length control apparatus in accordance with the present inventionas described above with first and second piezoelectric ceramic materialelements or stacked piezoelectric material elements may further includea mirror transducer substrate assembly having a light reflecting means.The mirror transducer substrate has a second outer rim member rigidlyaffixed to the first outer rim member of the base plate and a secondcentral member including the light reflecting means structured to movein tandem with the first central member of the base plate.

As employed in the present invention, the shapes of any of theelectrodes and piezoelectric ceramic material elements may bedisk-shaped, annular or donut-shaped, for example. In some aspects ofthe invention the number, surface areas and or cross-sectionalthicknesses of any of the electrodes or piezoelectric ceramic materialelements are selectively varied to provide a selected temperaturesensitivity of movement of the central member along the first centralaxis.

Other objects, features and advantages of the invention will be apparentthrough the detailed description, claims and drawings herein, whereinlike numerals refer to like elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically a cross-section of one example of a pathlength control driver assembly of the prior art.

FIG. 2 shows schematically a cross-section of one example of a pathlength control driver assembly fabricated in accordance with oneembodiment of the present invention.

FIG. 3 shows schematically a cross-section of another example of a pathlength control driver assembly fabricated in accordance with analternative embodiment of the present invention including a mirrortransducer substrate.

FIG. 4 shows schematically a top view the path length control driver ofFIG. 3.

FIG. 5 shows schematically an electrical circuit diagram of an exampleof a path length control driver assembly made in accordance with oneembodiment of the present invention including stacked piezoelectricmaterial elements.

FIG. 6 shows a top view of one example of an electrode employed in apath length controller made in accordance with the present invention.

FIG. 7 shows schematically a cross-section of a path length controllerof the invention having piezoelectric elements of differing thicknesses.

FIG. 8 shows schematically a cross-section view of a path lengthcontroller of the invention having differing numbers of piezoelectricceramic elements on opposite surfaces of a base plate.

FIG. 9 shows schematically a cross-section view of a path lengthcontroller according to an alternative embodiment of the inventionemploying an extended outer rim on the base plate member.

FIG. 10 shows yet another alternative embodiment of the inventionemploying a juxtapositioned pair of post members of varying thermalexpansion properties.

FIG. 11 shows yet another alternative embodiment of the inventionemploying electrodes of different thicknesses to create a selectedthermal sensitivity for tuning a path length control driver.

FIG. 12 shows graphically thermal movement of a ring laser gyro assemblyand a path length control driver in relation to temperature.

FIG. 13 shows an improved mirror transducer substrate for use with thepath length control driver of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

The examples and embodiments described herein are meant by way ofillustration of the techniques disclosed by the invention and are notintended to be limiting to the scope of the invention. FIG. 2 shows oneexample of a path length control driver and mirror transducer substrateas contemplated by the present invention.

The path length control driver 60 includes a base plate member 31generally having a cupped shape. The base plate member 31 includes acentral post member 32, an outer rim member 34, and an annular diaphragmmember 35 extending between and integral with central post member 32 andouter rim member 34. Extending from outer rim member 34 is a mountingflange 36. The base plate 31 may preferably be comprised of a lowthermal coefficient material such as, for example, invar or super invarmaterial. The first piezoelectric ceramic material 64 is mounted to thetop surface 66 of the base plate. The piezoelectric material 64 mayadvantageously have a disk or donut shape. A second piezoelectricceramic material 68, which may be generally donut-shaped is affixed tothe annular diaphragm member 35 on the opposite side of the top surface66. A first ring electrode 70 is rigidly affixed to the firstpiezoelectric ceramic material 64. A second ring electrode 72, of aselectively differing size and/or shape than the first ring electrode70, is rigidly affixed to the second piezoelectric ceramic material 68.The first and second electrodes advantageously have selected anddiffering shapes and sizes so as to provide a selected temperaturesensitivity of movement of the central member along the central axis 11.

Referring now to FIG. 3, a path length control driver assembly 30attached to a mirror transducer substrate 10 is shown. A doublediaphragm mirror assembly is illustrated. It is generally cylindricallyshaped and includes a top half 12 and a bottom half 20 being E-shaped incross section. Top half 12 includes a central post member 14, an outerrim member 15, and a thin annular diaphragm member 17 extending radiallybetween central post member 14 and outer rim member 15. Similarly,bottom half 20 includes a central post member 24, outer rim member 25and a thin annular diaphragm member 27 extending between central member24 and outer rim member 25. Outer rim member 25 includes a mountingsurface 26. Further, central post member 24 includes an external surface28 having either a mirror or reflective means 29, generally being amultilayer dielectric mirror.

In some cases the mirror transducer substrate 10 may comprise twoindividual piece parts, E-shaped in cross section, forming the top half12 and the bottom half 20 and joined at their interface 21.Alternatively, a cylindrical block may be provided with top and bottomtrepans to form diaphragm 27. In such a case the other diaphragm 17 isprovided by a disk and joined to the bottom half to create the doublediaphragm 17, 27. Another embodiment of the mirror transducer substrateassembly 10 may be a single block which is bored to provide the doublediaphragm member 17, 27 as illustrated.

Mirror transducer substrate assembly 10 is shown rigidly affixed to acorner of a ring laser gyro block 40 having a pair of interconnectingtunnels 42 and 44. Laser block 40 generally includes a mounting surface46 which is affixed to surface 26 of outer rim member 25 for providing agenerally gas tight seal therebetween.

Mirror transducer substrate assembly 10 is positioned on laser block 40such that laser beams 48 and 49 can reflect from mirror 29 in a wellknown manner. Path length control driver assembly 30 is comprised of agenerally cup-shaped driver fixture 31 including a central post member32, an outer rim member 34, and an annular diaphragm member 35 extendingbetween and integral with central post member 32 and outer rim member34. Extending from outer rim member 34 is a mounting flange 36 which issecured to the mirror assembly outer rim member 15 at surface 19thereof. Further, central post member 32 is affixed to central postmember 14 at surface 19 thereof. A plurality of circular donut-shapedpiezoelectric ceramic material elements 80A, 80B, 80C and 80D are alsoshown. A first plurality of at least two piezoelectric material elements80A and 80B is stacked and rigidly affixed to the top surface of baseplate member 31. A second plurality of piezoelectric ceramic materialelements 80C, 80D is stacked and rigidly affixed to the bottom surfaceof base plate 31 substantially surrounding central post 32. In oneembodiment of the invention, each of the individual piezoelectricmaterial elements is identical in size, shape and piezoelectric materialto the other piezoelectric elements. First, second and third electrodes82, 84, and 86, respectively are rigidly affixed to selected ones of thepiezoelectric ceramic material elements. A fourth electrode 87 (notshown in this view but shown in FIG. 5) is juxtaposed between the top ofthe base plate and element 80B. The electrodes may also advantageouslybe generally donut-shaped. A more detailed diagram of one example of theelectrode and piezoelectric ceramic material connections made inaccordance with the invention is shown schematically in FIG. 5 describedhereinbelow.

FIG. 4 shows a top view of the path length control driver of FIG. 3 ascontemplated by the present invention. The first electrode 82 is affixedto the flex tape 90 at terminal 102 which is, in turn, connected to afirst post terminal 92 by means of a first conductor 104 imbedded withinthe flex tape. The second electrode 84 is connected to a second terminal101 which is connected by means of a second conductor 106 imbeddedwithin the flex tape to a second post terminal 96. A third terminal 108is connected by means of a third conductor 110 imbedded within the flextape to a third terminal post 94. Terminal posts 92, 94 and 96respectively are connected to voltage potential sources as are furtherdescribed with respect to FIG. 5.

Referring now to FIG. 5, one embodiment of a detailed electrical circuitdiagram of the path length control driver 30 shown in FIGS. 3 and 4 isshown. First, second, third and fourth piezoelectric ceramic materialelements 80A, 80B, 80C and 80D are shown wherein elements 80A and 80Bare arranged to be affixed to the top of base plate 31 and elements 80Cand 80D are arranged to be affixed to the bottom of base plate 31.Interposed between piezoelectric ceramic elements 80A and 80B is thefirst electrode 82 which is connected to a positive potential El throughterminal post 92. A second electrode 84 is interposed and rigidly bondedbetween piezoelectric ceramic material elements 80C and 80D and furtherconnected to a second positive electrical potential E2 through terminalpost 96. The third electrode 86 wraps around the outer surfaces ofpiezoelectric ceramic material elements 80C and 80D. The fourthelectrode 87 is affixed between the second piezoelectric materialelement 80B and the top surface 66 of the base plate. A fifth electrode94A, which may advantageously comprise a wire in some cases, isconnected between the negative polarity side of piezoelectric element80A and terminal 94. The electrodes are rigidly bonded by well knownmeans such as electrically conductive epoxy to the various elements andthe base plate. All of the electrodes in this example may advantageouslybe generally ring or donut-shaped with the exceptions of the third andfifth electrodes 86,94A. The third electrode 86 is illustrated in FIG.6, and is comprised of two generally donut-shaped members 86A and 86Bconnected by a connecting member 91.

Referring now to FIG. 7 yet another embodiment of the inventionillustrating a thermal tuning apparatus utilizing a combination ofselected piezoelectric elements and electrode designs is shown. A firstpiezoelectric element 202 is mounted to a base plate 231 and a secondpiezoelectric material element 204 is mounted within a trepan 240 in thebase plate 231 surrounding a center post 233. A first ring electrode 236is rigidly affixed to the first piezoelectric ceramic material element202. A second ring electrode 238 is rigidly affixed to the secondpiezoelectric ceramic material 204. It has been found that by varyingthe thicknesses of the first and second piezoelectric ceramic materialsthermal tuning of path length within a ring laser gyro can becontrolled. In this case, the first piezoelectric ceramic material 202has a first selected thickness which is greater than a second selectedthickness of the second piezoelectric material element 204. Furthercontrol may be achieved by also varying the shapes, thicknesses and/orannular surface areas of the first and second ring electrodes 236, 238.That is, the inside and outside diameters of the donut-shaped electrodesmay be advantageously be non-symmetrical to selectively achieve adesired temperature sensitivity.

Referring now to FIG. 8, yet another alternative embodiment of theinvention is shown wherein a base plate 331 has a top surface 366. Afirst stack of a plurality of piezoelectric ceramic material elements380A and 380B is rigidly affixed to the top surface 366. A firstelectrode 382 is rigidly affixed between the first and secondpiezoelectric material elements 380A and 380B. A second electrode 387may be rigidly affixed between the top 366 of the base plate and thesecond piezoelectric material element 380B. As in the examples describedherein above a trepan 340 in base plate 331 allows placement of a thirdpiezoelectric material 380C which is of the same shape and size ofelements 380A and 380B and has yet a third electrode 383 rigidly bondedto it. In this way, by varying the number, of similarly sized and shapedpiezoelectric ceramic material elements, and/or by varying the number,shapes and sizes of electrodes it has been found that thermal tuning ofpath length control drivers can be achieved. Of course, the polarity ofcompensation caused by the differing number of piezoelectric elementsdepends on the location of the greater number or larger sizedpiezoelectric elements.

Referring now to FIG. 9 yet another alternative embodiment of a pathlength control driver system with thermal tuning is shown. The apparatusincludes a base plate 431 having an outer rim 455 and a central member450 surrounded by a top trepan 457. A bottom trepan 458 surrounds acentral post 432. In one example of the invention the central post 432may comprise a plurality of at least three stacked materials including afirst material 432A integral with the base plate having a firstcoefficient of thermal expansion, a second material 410 having yet asecond coefficient of thermal expansion, and a third material 412 havingyet a third coefficient of thermal expansion. The path length controldriver may also advantageously include piezoelectric elements 480 ofsimilar donut-shaped material in the top and bottom trepans. Anycombination of the features of the central member 450 and outer ring455, non-symmetrical top and bottom electrodes 436 and 486 and/orvarying materials on the center post 432 can be advantageously used toachieve thermal tuning of the path length control driver. The use ofdifferent materials having mismatched coefficients of thermal expansionin the center post 432 is explained in more detail hereinbelow withreference to FIG. 10.

Referring now to FIG. 10, yet another alternative embodiment of a pathlength control driver as contemplated by the invention and mounted to amirror transducer substrate is shown. The path length control drivercomprises a base plate 531 having a receiving aperture 511B. The baseplate 531 has a top surface 566 on which is mounted a piezoelectricmaterial element 580 which may advantageously be disk-shaped. The baseplate includes flanges 536 which are rigidly affixed to the mirrortransducer substrate 540. A juxtapositioned pair of post members 510,512 having different thermal expansion coefficients is located by meansof alignment member 511A within the receiving aperture 511B of the baseplate and rigidly affixed to a top member 520 of the mirror transducersubstrate 540. A second piezoelectric, donut-shaped material 582 isrigidly affixed to the underside of base plate 531. Piezoelectricceramic material element 582 forms an annular ring around thejuxtapositioned pair of post members 510 and 512. The juxtapositionedpair of post members 510 and 512 is located about a central axis denotedby arrow 611. The mirror transducer substrate 540 includes a centralpost member 570 including a mirror 29 which is rigidly affixed in linewith the central axis denoted by arrow 611. As the juxtapositioned pairof post members 510, 512 moves in a direction along arrow 611 mirror 29follows the movement of the post members in a tandem fashion. Thejuxtapositioned pair of post members 510, 512 are selected so as toyield a predetermined movement of the mirror 29 in a linear directiondenoted by arrow 611. In this way, thermal tuning of the driver andmirror transducer substrate is achieved.

FIG. 11 shows yet another alternative embodiment of the inventionwherein electrodes of different thicknesses are employed to achieve adesired temperature sensitivity of movement for the purpose of thermaltuning of a path length control assembly 631. A first electrode 670having a first selected thickness is rigidly affixed to a firstpiezoelectric element 664. A second electrode 672 having a secondselected thickness, which s different than the first selected thicknessof the first electrode, is rigidly affixed to a second piezoelectricelement 668. By varying the electrode thicknesses, selected temperaturesensitivities of movement of the central member 632 can be achieved.

Referring now to FIG. 13, an improved mirror substrate 1310 for use withthe path length control driver of the present invention is shown. Themirror substrate 1310 has a mirror side and a driver side and featuresan outer rim 1325 having a nonuniform thickness. The mirror side of themirror transducer substrate has an optical surface 1360. A mirror 29 isrigidly affixed to the optical surface 1360. A first trepan on thedriver side of the mirror transducer substrate has a first radius R5. Asecond trepan on the mirror side of the mirror transducer substrate hasa second radius R4. As indicated by the broken line 1350, R5 ispreferably a larger radius than R4 resulting in the nonuniform wallthickness of outer rim 1325. A cover member 1370 is rigidly affixed tothe outer rim and central post 1324 to form a diaphragm 1317. Thediaphragm 1317 is advantageously made thinner than previous designs. Incontrast to known designs, the more slender cylindrical portion of outerrim 1325 absorbs most of the thermal forces produced by a path lengthcontrol driver.

The mirror transducer substrate 1310 of FIG. 13, by departing fromuniform outer wall thicknesses used in known designs, reduces mirror andoptical surface deformation. In one example, improvements were achievedby changing dimension R5 from 0.200 to 0.240 inches. The resulting moreslender cylinder absorbs most of the thermal forces produced by the pathlength control driver and protects the optical surface from distortion.

It should be understood that various modifications may be made to theapparatus as disclosed herein without departing from the spirit andscope of the invention. Consequently, the present invention is notintended to be limited to the particular embodiment and apparatus shownin the drawings except as defined by the appended claims. For example,thermal compensation characteristics can also be modified by usingalternative materials for the path length controller base plate. As afurther example, varying the strength of the base plate rim or hoop mayalso advantageously vary the thermal characteristics of the assembly.This may be accomplished by, for example, breaking the hoop in aselected area, adding an alternative material around the hoop or varyingthe wall thickness of the base plate.

I claim:
 1. A path length control driver apparatus comprising:(a) afirst central member having first and second opposite ends; (b) a firstouter rim member; (c) a first flexible annular diaphragm area havingfirst and second opposite sides, the first flexible annular diaphragmarea being between and integral with the first central member and thefirst outer rim member; (d) a central axis passing centrally through thefirst and second opposite ends of the first central member; (e) a firstpiezoelectric means having first and second opposing sides wherein thefirst side of the first piezoelectric means is rigidly affixed to thefirst flexible annular diaphragm area's first side; (f) a secondpiezoelectric means having first and second opposing sides wherein thefirst side of the second piezoelectric means is rigidly affixed to thefirst flexible annular diaphragm area's second side; (g) a firstelectrode rigidly affixed to the second side of the first piezoelectricmeans; and (h) a second electrode rigidly affixed to the second side ofthe second piezoelectric means, wherein the first and second electrodeshaving selected shapes of different sizes to provide a selectedtemperature sensitivity of movement of the central member along thecentral axis.
 2. The path length control apparatus of claim 1 furthercomprising:(a) a mirror transducer substrate assembly for mounting alight reflecting means, the mirror transducer substrate assemblycomprising,a. a second central member having first and second oppositeends, b. a second outer rim member, c. a second flexible annulardiaphragm area having first and second opposite sides, the secondflexible annular diaphragm area being between and integral with thesecond central member and the second outer rim member, and the centralaxis passing centrally through the first and second opposite ends of thefirst and second central members, the first end of the second centralmember of the mirror transducer substrate assembly including a means forreflecting electromagnetic waves; and (b) means for rigidly couplingtogether the first and second outer rim members so as to cause the lightreflecting means to move in tandem with the first central member.
 3. Thepath length control apparatus of claim 1 wherein the first piezoelectricmeans is comprised of a first selected thickness and the secondpiezoelectric means is comprised of a second selected thickness which isdifferent from the first selected thickness.
 4. The path length controldriver apparatus of claim 1 wherein the selected shapes of the first andsecond electrodes comprise selected thicknesses of different sizes toprovide a selected temperature sensitivity of movement of the centralmember along the central axis.
 5. The path length control apparatus ofclaim 4 further comprising:(a) a mirror transducer substrate assemblyfor mounting a light reflecting means, the mirror transducer substrateassembly comprising,a. a second central member having first and secondopposite ends, b. a second outer rim member, c. a second flexibleannular diaphragm area having first and second opposite sides, thesecond flexible annular diaphragm area being between and integral withthe second central member and the second outer rim member, and thecentral axis passing centrally through the first and second oppositeends of the first and second central members, the first end of thesecond central member of the mirror transducer substrate assemblyincluding a means for reflecting electromagnetic waves; and (b) meansfor rigidly coupling together the first and second outer rim members soas to cause the light reflecting means to move in tandem with the firstcentral member.
 6. The path length control apparatus of claim 4 whereinthe first piezoelectric means is comprised of a first selected thicknessand the second piezoelectric means is comprised of a second selectedthickness which is different form the first selected thickness so as toprovide a further selected temperature sensitivity of movement of thefirst central member.
 7. The path length control apparatus of claim 1wherein the shapes of the first and second electrodes are selected fromone of the group consisting of a ring shape, a disk shape and a donutshape.
 8. The path length control apparatus of claim 1 wherein theshapes of the at least first and second piezoelectric means are selectedfrom one of the group consisting of a ring shape, a disk shape and adonut shape.
 9. A path length control apparatus comprising:(a) a firstcentral member having first and second opposite ends; (b) a first outerrim member; (c) a first flexible annular diaphragm area having first andsecond opposite sides, the first flexible annular diaphragm area beingbetween and integral with the first central member and the first outerrim member; (d) a central axis passing centrally through the first andsecond opposite ends of the first central member; (e) a first stackcomprising a first plurality of piezoelectric means comprising a numberof elements, the first stack having first and second opposing sideswherein the first side of the first stack of piezoelectric means isrigidly affixed to the first flexible annular diaphragm area's firstside; (f) a second stack comprising a second plurality of piezoelectricmeans comprising a number of elements, the second stack having first andsecond opposing sides wherein the first side of the second stack ofpiezoelectric means is rigidly affixed to the first flexible annulardiaphragm area's second side; and (g) at least first and secondelectrodes wherein the first electrode is rigidly affixed to the secondside of the first stack of piezoelectric means, wherein the secondelectrode is rigidly affixed to the second side of the second stack ofpiezoelectric means, and wherein the at least first and secondelectrodes have selected shapes to provide a selected temperaturesensitivity of movement of the central member along the first centralaxis.
 10. The path length control apparatus of claim 9 furthercomprising:(a) a mirror transducer substrate assembly for mounting alight reflecting means, the mirror transducer substrate assemblycomprising,a. a second central member having first and second oppositeends, b. a second outer rim member, c. a second flexible annulardiaphragm area having first and second opposite sides, the secondflexible annular diaphragm area being between and integral with thesecond central member and the second outer rim member, and the centralaxis passing centrally through the first and second opposite ends of thefirst and second central members, the first end of the second centralmember of the mirror transducer substrate assembly including a means forreflecting electromagnetic waves; and (b) means for rigidly couplingtogether the first and second outer rim members so as to cause the lightreflecting means to move in tandem with the first central member. 11.The path length control apparatus of claim 9 wherein the number ofelements in the first plurality of piezoelectric means is not equal tothe number of elements in the second plurality of piezoelectric means.12. The path length control apparatus of claim 9 wherein the firstplurality of piezoelectric means is comprised of piezoelectric materialelements which have a first selected thickness and the second pluralityof stacked piezoelectric means is comprised of piezoelectric materialelements which have a second selected thickness which is different fromthe first selected thickness so as to provide a further selectedtemperature sensitivity of movement of the central member.
 13. The pathlength control apparatus of claim 9 wherein the selected shapes of theat least first and second electrodes comprise selected thicknesses ofdifferent sizes to provide a selected temperature sensitivity ofmovement of the central member along the first central axis.
 14. Thepath length control apparatus of claim 9 wherein the number ofelectrodes mounted on the first end of the first central member isdifferent than the number of electrodes mounted to the second end of thefirst central member.
 15. The path length control apparatus of claim 9wherein the shapes of the at least first and second electrodes areselected from one of the group consisting of a ring shape, a disk shapeand a donut shape.
 16. The path length control apparatus of claim 9wherein the shapes of the at least first and second piezoelectric meansare selected from one of the group consisting of a ring shape, a diskshape and a donut shape.
 17. The path length control apparatus of claim13 further comprising:(a) a mirror transducer substrate assembly formounting a light reflecting means, the mirror transducer substrateassembly comprising,a. a second central member having first and secondopposite ends, b. a second outer rim member, c. a second flexibleannular diaphragm area having first and second opposite sides, thesecond flexible annular diaphragm area being between and integral withthe second central member and the second outer rim member, and thecentral axis passing centrally through the first and second oppositeends of the first and second central members, the first end of thesecond central member of the mirror transducer substrate assemblyincluding a means for reflecting electromagnetic waves; and (b) meansfor rigidly coupling together the first and second outer rim members soas to cause the light reflecting means to move in tandem with the firstcentral member.
 18. The path length control apparatus of claim 13further comprising at a least third electrode, wherein the thirdelectrode is affixed between the first side of the first annulardiaphragm and the first side of the first stack.
 19. The path lengthcontrol apparatus of claim 13 wherein the number of elements in thefirst plurality of piezoelectric means is not equal to the number ofelements in the second plurality of piezoelectric means.
 20. The pathlength control apparatus of claim 13 wherein the first plurality ofpiezoelectric means is comprised of piezoelectric material elementswhich have a first selected thickness and the second plurality ofpiezoelectric means is comprised of piezoelectric material elementswhich have a second selected thickness which is different from the firstselected thickness so as to provide a further selected temperaturesensitivity of movement of the first central member.
 21. A path lengthcontrol apparatus comprising:(a) a base plate having a first alignmentmeans, mounting means, an underside and a top surface; (b) piezoelectricmeans rigidly mounted to the top surface of the base plate; (c) at leastfirst and second post members having differing thermal expansioncoefficients, and having second and third alignment means, respectively,wherein the first, second and third alignment means cooperate to alignthe first and second post members with the base plate to form a centralpost having top and bottom opposing ends; (d) a second piezoelectricmeans rigidly affixed to the underside of the base plate; and (e) amirror transducer substrate assembly rigidly affixed to the base platemounting means and the bottom end of the central post.
 22. The apparatusof claim 21 wherein the first and second piezoelectric materials have aselected shape selected from one of a disk shape and a donut shape. 23.A path length control apparatus comprising:(a) an outer rim memberhaving an inner flexible diaphragm connected to a central member, thecentral member having first and second opposite ends, such that when theinner flexible diaphragm moves the central member moves in relation tothe outer rim member along a central axis passing centrally through thefirst and second opposite ends of the central member; (b) a first stackcomprising a first plurality of piezoelectric means comprising a numberof elements, the first stack having first and second opposing sides; (c)a second stack comprising a second plurality of piezoelectric meanscomprising a number of elements, the second stack having first andsecond opposing sides; (d) a first electrode juxtaposed between two ofthe piezoelectric means in the first stack; (e) a second electrodejuxtaposed between the first side of the first stack of piezoelectricmeans and the first flexible annular diaphragm area's first side; (f) afirst electrode having a first member juxtaposed between the first sideof the second stack and the second side of the first flexible annulardiaphragm area having a second connected member rigidly affixed to thesecond side of the second stack; (g) a fourth electrode juxtaposedbetween two of the plurality of piezoelectric means in the second stack;and (h) wherein the first, second, third and fourth electrodes haveselected shapes to provide a selected temperature sensitivity ofmovement of the central member along the first central axis.